Refinery and petrochemical plants have many processes where hydrocarbon containing various amounts of water is heated to vapor and, later in the process, is condensed back to a liquid state. Many of these process contain or generate one or more corrosive components. These corrosive components may include strong acids such as hydrochloric, sulfuric, or phosphoric acid and/or weak acids such as hydrogen sulfide, carbon dioxide, and organic acids. When the vapors in these units condense, water containing one or more corrosives attacks the internal surfaces of the condensing sections. The exact point where corrosion begins, the nature, and the severity of the corrosion attack cannot be predicted accurately without making actual measurements.
Heretofore, a number of devices and techniques have been used to monitor corrosion, but the accuracy of these devices and techniques has been limited. For example, ultrasonic equipment has been used to measure with sound waves the metal thickness of equipment. This method is limited because the corrosion process is often localized to only a portion of the equipment and this area can be missed in testing or isolated to an area that is inaccessible to testing.
Weight loss coupons have been used to detect and measure corrosion in these units. The corrosion rates determined from the use of a coupon only represents the time weighted average of corrosion that occurred at the point of the coupon's installation. In order to predict the corrosion rate of a refinery or petrochemical unit, it is known that the most severe corrosion will occur at a point at or near the point of initial water condensation referred to as the dew point. In this equipment, it has been learned that this point is most typically inside a heat exchanger or a condenser. This area of highest corrosion inside a heat exchanger can be shifted as the unit's vapor temperature and pressure shifts and both the location and the shifting of the corrosion makes it impossible to maintain a coupon in this area.
Electrical resistance probes are considered to be the most accurate device to monitor refinery and petrochemical equipment corrosion and these devices are superior to corrosion coupons as the time weighting factor can be greatly reduced to measure changes in corrosion rates over a much shorter interval. The electrical resistance probes do, however, share the weakness with corrosion coupons of limited access and a shifting corrosion pattern.
Sampling the process stream followed by chemical testing for corrosive agents and corrosion by-products has proven useful in estimating corrosion. These techniques, however, do not yield much useful information on the area or type of corrosion and the sensitivity is poor.
There is an apparatus for measuring corrosion potential which analyzes the initial condensate condition where the temperature of the surrounding environment reaches the dew point of water, as disclosed in U.S. Pat. No. 3,649,167. However this system will not indicate metal loss or corrosion activity.
It is also known that there is an apparatus which includes a water box having a coil arranged in the box through which hydrocarbon vapors are directed. Cooling water is forced through the box in a counterflow direction to cool the vapor and simulate the cooling that will take place in heat exchanger units. Corrosion and temperature probes are arranged along the coil to determine the corrosion rates at various temperature levels of the hydrocarbon. This device is disclosed in U.S. Pat. No. 4,335,072. However, this system has several restrictive limitations. The unit must be set to only one temperature by adjusting the cooling water flow and/or process flow. The temperature profile established is both uncontrollably fixed and individually unadjustable. The cooling water box imposes strict limits on the volume of process flow required and the amount of temperature change desired. This unit also has no rapid or accurate method to determine the process flow dew point which identifies the critical temperature range for profiling corrosion.